To assess the unique properties of low-temperature (LT) grown Si on which high-quality SiGe pseudo-substrates were obtained, positron annihilation spectroscopy (PAS) measurements were done for Si layers deposited at growth temperatures between 200 degreesC and 700 degreesC by molecular beam epitaxy before and after annealing between 400 degreesC and 700 degreesC. PAS showed, for the first time, that LT-SI possessed extremely large vacancy clusters with very high-density (open space greater than 10 atoms, greater than or equal to 10(18) cm(-3)). Since these vacancy clusters are considered to weaken the mechanical strength of the thin LT-SI layer, the formation process of high-quality SiGe pseudo-substrate on LT-Si was understood by using a model of a strained compliant substrate/epitaxial film system.